1. Field of the Invention
The present invention relates to an image processing apparatus, a printing apparatus and an image processing method, and more particularly, to image processing for reducing a density unevenness caused by ink ejection volume variations among a plurality of nozzles that eject ink.
2. Description of the Related Art
A printing head used in an inkjet printing apparatus may have ink ejection volume variations among a plurality of nozzles due to, for example, a manufacturing error. Such ink ejection volume variations are likely to cause a density unevenness in a printed image.
It has been well known to employ head shading technique as described in Japanese Patent Laid-Open No. 1110-013674 (1998) in order to reduce such a density unevenness. The head shading corrects image data, based on information about an ink ejection volume of each nozzle. This correction increases or decreases the number of ink dots eventually printed, thereby adjusting a density of the printed image.
However, even by using the head shading, when a certain color is represented by overlapping two or more inks, there may occur a color difference, that is, a color printed with nozzles having ink ejection volume variations is different from the color that should be printed. For example, when a blue image is printed by the dots of cyan ink and the dots of magenta ink, the dots of magenta ink are formed in bigger size than a standard size in a region where a magenta ink ejection volume is more than a standard volume due to the variations. Along with this, the number of the dots is less than a standard number by correction of head shading. As a result, cyan dots, as well as dots each being composed of a blue area and a magenta area surrounding the blue area exit in this region. A color of this region is different from the color that should be printed, that is, the color of the region where blue dots are printed by cyan and magenta ink without ink ejection volume variations.